A. Field of the Invention
The field of the present invention is wireless (radio) communications. In particular, the field is using antenna arrays and spatial signal processing in wireless communications systems to determine parameters of a communication system such as frequency offset, time alignment and an initial weight vector for spatial processing.
B. Background
Wireless communications systems
Users of a wireless communications system typically access the system using remote terminals such as cellular telephones and data modems equipped with radio transceivers. Such systems generally have one or more radio base stations, each of which provides coverage to a geographic area known as a cell. The remote terminals and base stations have protocols for initiating calls, receiving calls, and general transfer of information.
In such a system, an allocated portion of the spectrum is divided up into communication channels which may be distinguished by frequency, by time, by code, or by some combination of the above. Each of these communication channels will be referred to herein as a conventional channel. To provide full-duplex communication links, typically some of the communication channels are used for communication from base stations to users' remote terminals (the downlink), and others are used for communication from users' remote terminals to base stations (the uplink). Within its cell, a radio base station can communicate simultaneously with many remote terminals by using different conventional communication channels for each remote terminal.
We have previously disclosed spatial processing with antenna arrays to increase the spectrum efficiency of such systems. See U.S. patent applications: Ser. No. 07/806,695 filed Dec. 12, 1991, entitled Multiple Access Wireless Communications Systems (also U.S. Pat. No. 5,515,378 issued May 7, 1996); Ser. No. 08/234,747, filed Apr. 28, 1994, entitled Method and Apparatus for Calibrating Antenna Arrays (now U.S. Pat. No. 5,546,909 issued Aug. 13, 1996); Ser. No. 08/283,470, filed Aug. 1, 1994, entitled Spectrally Efficient and High Capacity Acknowledgment Radio Paging System; and Ser. No. 08/375,848, filed Jan. 20, 1995, entitled Spectrally Efficient High Capacity Wireless Communications Systems (collectively, "Our Co-pending Patent Applications"). The general idea is to increase the quality of communication by using an antenna array rather than a single antenna, together with processing of the signals received at the antennas. The antenna array also can be used to increase spectrum efficiency by adding spatial multiplexing to conventional channels so that several users can communicate simultaneously on the same conventional channel. We call this SDMA for spatial division multiple access. Thus, taking frequency division multiplexing (FDMA) as an example, with SDMA, several remote terminals may communicate with one or more base stations on a single cell on the same frequency channel, that is, on the same conventional channel. Similarly, with time division multiplexing (TDMA) and SDMA, several remote terminals may communicate with one or more base stations on a single cell on the same frequency channel and the same time slot, that is, on the same conventional channel. SDMA likewise also can be used with code division multiple access (CDMA).
Parameters of a Communication System
Frequency offset and time alignment
It is often required to estimate certain parameters of a communication system such as frequency offset and time alignment. The frequency offset problem can be described as follows. In a typical radio-frequency (RF) receiver, the original RF signal is mixed down using local frequency references, typically produced by crystal oscillators and/or frequency synthesizers, to produce a baseband signal whose phase and amplitude changes around in a predictable pattern determined by the modulation format. Ideally, the signal has no residual frequency offset component, such an offset due for example to frequencies of the local oscillators differing slightly from the frequency of the oscillators used in sending the signals. In the case of mobile communications transmitting from a handset to a base station, the frequency of the radio signal is produced by a local oscillator in the hand set, while the frequency references used for down-converting the signal are produced by different local oscillators in the base station. Although the base station local oscillators typically are very good, there still typically is frequency offset in the residual is signal. In order to increase system performance, it is desirable to estimate this frequency offset and correct for it, for example, in demodulation. Prior art techniques for frequency estimation include simple DC filtering of the incoming signal. Other prior art techniques include taking some high power of the incoming signal, for example the fourth power. As an example, with .pi./4 differential quaternary (or quadrature) phase shift keying (.pi./4 DQPSK), exponentiating the incoming signal to the power 4 gives all of the complex points in the constellation falling back on each other on the phase plane, and thus the DC value of the fourth power of the signal gives one an estimate of the frequency offset. The problem with these and other similar techniques is that they are not sufficiently robust in the face of noise or in the presence of significant amounts of interference on the signal input. In many situations, in particular in the case of cellular communications systems, the interference may be signals that are from other sources in the same communications system, so have the same modulation format. Such interference is one of a variety of possible interference from other signals on the same channel, so is called co-channel interference. Prior art techniques for estimating the frequency offset parameter do not in general work well when one has a low carrier to interference ratio (C/I) as is the case when one has strong co-channel interference.
In order to best demodulate digitally modulated signals, it is known that one needs to estimate the time alignment parameter of the incoming signal. This involves determining exactly when in time the incoming signal, viewed on the complex plane, passes through the constellation points. That is, it involves synchronizing the initial timing of the symbols in the signals received with the timing of the signals sent. There are a number of techniques in the prior art for performing time alignment estimation. Such techniques often use known training sequences that are incorporated in the burst of interest. These training sequences are chosen to have particular correlation (or convolution) properties. A correlation (or convolution) operation can then be used to determine timing offset, as is known in the art. The problem with such prior art techniques is that they do not perform well in the presence of high co-channel interference. Two references for prior art techniques for time alignment and for frequency offset correction/estimation are: 1) Chapter 16,"Carrier Recovery" and Chapter 17, "Timing Recovery" in E. A. Lee and D. G. Messerschmitt, Digital Communications, 2nd Edition, Kluwer Academic Publishers, 1994; and 2) L. E. Franks, "Synchronization Subsystems: Analysis and Design," in K. Feher (Ed.), Digital Communications: Satellite/Earth Station Engineering, Prentice-Hall, Inc., 1983.
Thus, there is a need in the art for techniques for finding the time alignment and frequency offset parameters of a communication system that work well in the presence of strong co-channel interference.
Our Demodulation Invention, discloses a method and apparatus for improving reception and demodulation by augmenting the wireless communications system with multiple antennas, thereby introducing multiple versions of each signal, each of these versions comprising the composite of all the co-channel signals together with interference and noise. Our Demodulation Invention exploits the fact that the signal of interest has a particular modulation format by forcing estimates of signals of interest to more closely match the particular modulation format. Techniques of this nature sometimes are called decision directed or property restoral. Our Demodulation Invention in addition corrects for frequency offset and time alignment on an ongoing basis. Overall, it is designed to work well in the presence of strong co-channel interference. During establishing communications, Our Demodulation Invention uses an initial estimate of frequency offset and time alignment. Other communication systems that use arrays also require an initial estimate of frequency offset and time alignment.
Thus there is a need in the art for techniques for estimating the initial values of parameters of a communication system, such as the initial time alignment and initial frequency offset, that work well in the presence of strong co-channel interference and that are applicable to communication systems that use arrays of antennas.
Initial weight vector calculation
For communication systems that use arrays of antennas, another parameter that needs to be estimated is the initial weight vector defined below. Our Co-Pending Applications, Our Demodulation Invention, and other "smart antenna" techniques augment a wireless communications system with multiple antennas. In general m signals are received at m antennas. Spatial processing of the (complex valued) m signals at the m antennas comprises for each signal of interest determining a weighted sum of the antenna signals. The complex valued weights can be represented by a vector called herein a weight vector. The more general situation is that the received antenna signals need also to be temporally equalized, and in that situation, rather than a weighted sum, for each signal of interest, a sum of convolutions of the antenna signals is determined. That is, the weight vector is generalized, for the linear time invariant equalization situation, to a vector of complex valued impulse responses. For the purposes of this invention, the term weight vector shall apply either to a vector of complex weights or to a vector of impulse responses, depending on whether equalization is included.
Our Co-Pending Applications, Our Demodulation Invention, and other "smart antenna" techniques use a variety of methods to determine the weight vector on an ongoing basis. In each of these, the initial weight vector, a parameter of the system, also needs to be determined, and several methods have been proposed in the prior art for such initial weight determining. These include using ESPRIT or MUSIC to determine spatial signatures, then using these to determine the initial weight. It also includes maximum ratio combining and (3) principal component copy techniques, and using such techniques gives a starting weight that causes convergence upon the strongest signal. Thus, if the goal is to always pick out the strongest signal from a set of interferers, then such techniques work fine. However, such prior art techniques do not in general work well when one has a low carrier to interference ratio (C/I) as is the case when one has strong co-channel interference.
Thus, there is a need in the art for techniques for estimating the initial value of the initial weight vector parameter of a communication system with an antenna array that work well in the presence of strong co-channel interference.